Electronics > RF, Microwave, Ham Radio

DIY RF EMC Biconic Antenna

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dazz1:
Hi
My latest DIY project.
I am making a biconic antenna for EMC pre-compliance testing.  I am actually making two identical antenna so I can calibrate them against each other.
With any calibration, a reference is required.  I have recently had my restored Wavetek 2025 output calibrated to within 0.1dB.  My starting reference will be a hard wired (coax) connection between the signal generator  and the spectrum analyser. 
I will then replace the coax with two identical antenna separated by an exact distance, and orientated the same way relative to each other.  I can then calculate the gain compared to a pair of isotropic antenna (which only exist in text books).
I simply compare the measured gain of the antenna to calibrate them.
 
Making two antenna is only a small increment in effort.  Making two identical antenna will be faster than making dipoles etc.    I have spent a lot of time with the detailed design.  Much of the time required to make these is in the setup for each machining operation. 


The approach means I will end up with two identical calibrated antenna.  I only need one.

The first step is to make the top hat.  This is the piece that all of the antenna arms and the centre element connect at. 
Each arm will be held in place with a ball and spring detent.  The arms will snap into place.  This will make assembly and storage very easy.

The cross section shows the interior detail of the top hat. The antenna arms will slide into the top had and be held in place with the detents. 
I took a lot of care in the marking out to ensure accurate location of the holes.
The setup ensures that every part is identical.  I have 24 radial holes to drill and 24 opportunities to screw up.  The setup will reduce the risk of error.



mag_therm:
Hi dazz,
To make bicycle spoke hubs on the lathe I made a tool post drill spindle with a small dc motor. The indexing was by a 64 tooth gear on the back of the lathe main spindle with a tooth shaped pawl. That gear allows only 2, 4, 8, 16, 32, 64 holes on pcd.

artag:
When spacing them an exact distance apart, do you measure to the centreline or to the nearest point, biconics being quite large  ?

nctnico:
Interesting project!
Recently I have been testing with antennas as well for EMC testing (I bought a relatively cheap discone antenna which is also wideband) but I quickly found out that there is too much junk floating around in the air already to make any sensible measurement. IOW: I'm receiving so much noise already that it swamps what I want to measure. I came to the conclusion that I'd need a shielded room in order to have any chance of making a meaningfull measurement. Do you have a shielded chamber?


Also: how large is your antenna going to be? I looked at the $500-ish biconical anntenna from Tekbox but at the lower end (30MHz to 100MHz) it has very poor reception due to the relatively small size.

coppercone2:
Nice antenna. I have the parts needed for a balun, but I need to make the enclosure (I made a antenna at the wrong impedance, I am not sure I measured it right but it looks like I got like 130 ohms. The balun is for me the hard part of these antennas. I tested them in the basement and it looks like under 1GHz its pretty crowded.

Also have you considered a pre-selector for broad band antenna work? They always have a preselector in the RF labs when they use broad band antennas on the compliance analyzer machine (I think those machines are built with a preselector). Narrow band antenna does the work for you with the filtering.. broad band requires extra hardware I guess

For a hobbyist that collapsible design of the antenna you made in is very important. They are rather imposing on the laboratory.

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